Memory manufacturers are currently researching and developing the next generation of memory devices. One such development includes technology designed to replace current Flash non-volatile memory technology. Important elements of a Flash successor include compactness, low price, low voltage operation, non-volatility, high density, fast read and write cycles, and long life.
Current Flash technology is predicted to survive into 90 nanometer and 65 nanometer process generations. This survival is in part based on, for example, exotic storage dielectric material, cobalt and nickel source and drain regions, copper and low dielectric constant materials for the interconnect levels, and high dielectric constant materials for transistor gates. However, there will thereafter exist a need for new memory materials and technology, particularly for non-volatile memory.
Ferroelectric memory is one such technology aimed to replace Flash memory. A ferroelectric memory device combines the non-volatility of Flash memory with improved read and write speeds. Simply stated, ferroelectric memory devices rely on the use of ferroelectric materials that can be spontaneously polarized by an applied voltage or electric field and that maintain the polarization after the voltage or field has been removed. As such, a ferroelectric memory device can be programmed with a binary “1” or “0” depending on the orientation of the polarization. The state of the memory device can then be detected during a read cycle.
Two crystalline materials have emerged as promising films utilized in a ferroelectric memory scheme, namely lead zirconium titanate (“PZT”) and strontium bismuth tantalite (“SBT”). However, while the materials exhibit appropriate ferroelectric properties, each is nevertheless expensive to integrate into an existing CMOS process flow.
More recent developments include the use of polymers that exhibit ferroelectric properties. The creation of polymer ferroelectric memory utilizes polymer chains with permanent ionic dipole moments. Data is stored by changing the polarization of the polymer chain between metal lines that sandwich the layer comprised of the ferroelectric polymer chain. Further, the layers can be stacked (e.g., metal word line, ferroelectric polymer, metal bit line, ferroelectric polymer, metal word line, etc.) to improve memory element density. The polymer ferroelectric memory devices exhibit microsecond initial read speeds coupled with write speeds comparable to Flash.